Diabetes mellitus comprises a group of syndromes characterized by an inability of the body to produce adequate insulin or to properly use insulin. Most diabetes patients can be classified clinically as having either insulin-dependent diabetes mellitus (IDDM) or non-insulin-dependent diabetes mellitus (NIDDM). Nearly all forms of diabetes mellitus result from either a decrease in the secretion and blood concentration of insulin or a decrease in the response of tissues to insulin (insulin resistance), often associated with an elevated level of hormones (e.g., glucagon) that act contrary to insulin. Such abnormalities give rise to changes in carbohydrate, lipid and protein metabolism. The syndrome's hallmark is hyperglycemia; other complications can include cardiovascular disease, retinopathy, neuropathy, nephropathy, skin disorders and gastroparesis.
Diabetes mellitus affects millions of persons worldwide, including over 18 million in the United States. It is estimated that IDDM (Type I diabetes), which results from the body's failure to produce insulin, accounts for 5-10% of the cases of diabetes diagnosed in the United States. The majority of diabetes patients in the United States are diagnosed with NIDDM (Type II diabetes), which results from insulin resistance combined with the inability of the pancreas to secrete sufficient insulin to overcome such resistance. Type II diabetes occurs in at least 5% of the United States population, and in 1996 alone NIDDM affected 16 million people (Roman, S. H.; Harris, M. I., Endocrinology and Metabolism Clinics of North America, 1997, 26.3, 443-474). Impaired glucose tolerance (IGT), a syndrome characterized by impaired glucose processing that presents symptoms similar to a mild form of Type II diabetes, is even more prevalent, affecting 35 to 40 million adults in the United States.
Diabetes is most frequently diagnosed either by the presentation of a fasting plasma glucose of greater than or equal to 126 mg/dL on two occasions, or by an oral glucose tolerance test (OGTT) with a 2 hour post load value of greater than 200 mg/dL plus classic symptoms such as polydipsia, polyphagia and/or polyuria (The Expert Committee on the Diagnosis and Classification of Diabetes Mellitus, Diabetes Care, 1998, 21, S5-19). In the case of IGT, a fasting plasma glucose of less than 126 mg/dL but 2 hour post-oral glucose challenge lever greater than 140 mg/dL is observed.
A primary goal in the treatment of each of these conditions is the reduction and control of blood glucose levels. The reduction of hyperglycemia in insulin-dependent diabetes (IDDM) can attenuate the development of many of the attendant complications of IDDM (Diabetes Control and Complications Trial Research Group, New England J. Med., 1993, 329, 977-986). For example, tight control of blood glucose levels through intensive insulin therapy can reduce the development of retinopathy, nephropathy and neuropathy by >50% each in IDDM patients. These findings, together with the similarity of the pathologies seen in IDDM and NIDDM, suggest that control of blood glucose levels would produce similar benefits in NIDDM patients (American Diabetes Association, Diabetes Care, 1998, 21, S88-90), as has been reported (Ohkubo, Y., et al., Diabetes Res. Clin. Pract. 1995, 28, 103-117).
Several methods to treat hyperglycemia have been attempted. Patients with Type I diabetes receive insulin. In patients with Type II diabetes, the pancreas secretes insulin, but in insufficient amounts to overcome the intrinsic insulin resistance of the disease. The administration of agents such as metformin (De Fronzo, R. A.; Goodman, A. M. N. Engl. J. Med., 1995, 333, 541-549; Bailey, C. J. Biguanides and NIDDM, Diabetes Care 1992, 15, 773-784) and the glitazones (PPAR agonist class of drugs; Willson, T. M., et al., J. Med. Chem. 1996, 39, 665-668) can at least partially ameliorate insulin resistance, but these agents do not promote insulin secretion. Treatment with certain sulfonylureas has been shown to promote insulin secretion by affecting an ion channel; however, the increase in insulin caused by this class of drugs is not glucose dependent or even glucose sensitive, and such treatment can actually raise the risk of overt hypoglycemia. DPPV inhibitors, such as GLP or a GLP mimetic (such as Exedin), promote cAMP secretion at the β-cell through an incretin mechanism, and administration of these agents promotes insulin release in a glucose dependent manner (Vahl, T. P., D'Alessio, D. A., Expert Opinion on Invest. Drugs 2004, 13, 177-188). However, even with these potential treatments, it is difficult to achieve tight control of blood glucose levels in NIDMM patients in accordance with the guidelines recommended by the American Diabetes Association. Accordingly, there is significant demand for novel therapeutic approaches that allow sufficient glycemic control.
Possible approaches to glycemic control include enhancing clearance of glucose from the blood and increasing the rate of glucose storage or utilization. Glucose enters most cells by a specific transport protein, where it is phosphorylated to form glucose-6-phosphate in a reaction catalyzed by a hexokinase. Inside the cell, glucose-6-phosphate has one of several fates: it can be broken down via the glycolytic pathway, converted into glycogen or it can be oxidized via the pentose phosphate pathway.
Glucokinase (GK) (ATP: D-hexose 6-phosphotransferase), one of the four types of mammalian hexokinases (hexokinase IV), plays an essential role in blood glucose homeostasis. Expression of glucokinase is largely localized in the liver and pancreatic β-cells, where several types of glucokinase are expressed: these types differ in the sequence of the 15 N-terminal amino acids due to differences in splicing, but their enzymatic properties are virtually identical. Glucokinase is also expressed in a population of neurones in the hypothalamus.
Unlike the enzymatic activities of the other three hexokinases (I, II, III), each of which becomes saturated at a glucose concentration of below 1 mM, glucokinase has a Km for glucose of 8 mM, which is close to the physiological glucose level (5 mM). Thus, at lower glucose levels, glucose is more rapidly utilized in brain, muscle and other peripheral tissues—through conversion by a hexokinase other than glucokinase—than in the liver. At elevated glucose levels, such as after a meal or overnutrition (the postprandial glucose level can exceed 10-15 mM), glucokinase-mediated glucose metabolism in the liver and pancreas is accelerated. Moreover, hexokinases I, II and III are inhibited by high concentrations of glucose-6-phosphate, lowering glucose utilization, whereas glucokinase continues to catalyze utilization of glucose even at high levels of glucose-6-phosphate.
In tissues where glucokinase is expressed, it plays an important role in glucose uptake and utilization: in the β-cell, the glucose-6-phosphate produced is a necessary signal for insulin release; in the hypothalamus glucose-6-phosphate acts as a satiety signal and might contribute to the secretion of enteroincretins; and in the liver, where glucose-6-phosphate production by the action of glucokinase acts as a mechanism for disposal of excessive glucose through storage as glycogen (Printz, R. L., et al., Annu. Rev. Nutr., 1993, 13, 463-496). Glucokinase-catalyzed glucose phosphorylation is the rate-limiting reaction for glycolysis in hepatocytes and pancreatic β-cells. In the liver, glucokinase determines the rates of both glucose uptake and glycogen synthesis, and it is also thought to be essential for the regulation of various glucose-responsive genes (Girard, J., et al., Annu. Rev. Nutr., 1997, 17, 325-352). In both liver and pancreatic β-cells, glucokinase is rate limiting for glucose utilization, and consequently is a major component of the regulation of insulin secretion from the β-cell and glycogen storage in the liver. The control of insulin secretion and the control of glycogen storage are deficient in diabetes (DeFronzo, R. A., Diabetes, 1988, 37, 667-687).
The theoretical importance of glucokinase in diabetes is supported by studies of genetic populations and genetic manipulation of animal models of NIDDM. Mutation of glucokinase to a less active form of the kinase is the cause of the Maturity Onset of Diabetes in the Young (MODY-2) (Froguel, P., et al., New England J. Med., 1993, 328, 697-702; Bell, G. I., et al., Annual Rev. of Physiol., 1996, 58, 171-186). Conversely, humans with a glucokinase activation mutation are less prone to hyperglycemia and have increased insulin secretion in response to a glucose challenge (Christesen, H. B., et al., Diabetes, 2002, 51, 1240-1246; Gloyn, A. L, et al., Diabetes, 2003, 52, 2433-2440; Glaser, B., et al., New England J. Med., 1998, 338, 226-230). Also, NIDDM patients have been reported to have inappropriately low glucokinase activity. Furthermore, over expression of glucokinase in dietary or genetic animal models of diabetes either prevents, ameliorates, or reverses the progress of pathological symptoms in the disease (Caro, J. F., et al., Hormone & Metabolic Res., 1995, 27, 19-22). For these reasons, compounds that activate glucokinase have been sought by the pharmaceutical industry.
Substituted benzyl carbamoyl, substituted heterobenzyl carbamoyl, substituted phenyl carbamoyl, and substituted heteroaryl carbamoyl compounds have been disclosed as glucokinase activators. See, for example, WO 03/000267, WO 03/015774, WO 04/045614, WO 04/046139, WO 05/04480, WO 05/054200, WO 05/054233, WO 05/044801, WO 05/056530, WO 03/080585, WO 04/076420, WO 04/081001, WO 04/063194, WO 04/050645, WO 03/055482, WO 04/002481, WO 05/066145, WO 04/072031, WO 04/072066, U.S. Pat. No. 6,610,846, WO 00/058293, WO 03/095438, WO 01/44216, WO 01/083465, WO 01/083478, WO 01/085706, WO 01/085707, WO 02/008209, WO 02/014312, WO 02/046173, WO 02/048106, WO 03/095438, WO 04/031179, and WO 04/052869. These compounds either lower the Km for glucose and/or increase the Vmax of glucokinase. A class of glucokinase activators that can lower the Km of glucose moderately to 2-5 mM at low activator concentrations is desirable.